The present invention relates to improvements made to the cooling sections of lines for the continuous processing of metal strip, in particular annealed, galvanized or tin strip.
A line for the continuous processing of metal strip consists of a succession of thermal processing stations, in particular sections for heating, maintaining temperature, cooling, ageing etc.
The present invention relates to the cooling sections of continuous processing lines and more particularly sections for rapid cooling with the spraying of a liquid onto the strip.
The cooling liquid is generally water, which may be treated in advance, for example so as to extract the dissolved oxygen or the mineral salts therefrom, and which may contain additives for improving the thermal exchange or limiting the oxidation of the strip.
Cooling using water makes it possible to obtain very high cooling slopes, higher than those which may be obtained with gaseous cooling.
The cooling of the strip can also be obtained by spraying the strip with a mixture consisting of a gas and a liquid. In this case, the gas is generally present as a carrier gas for effecting the atomization and the spraying of the liquid onto the strip. The gas used is usually nitrogen but can also consist of a mixture of nitrogen and hydrogen, or any other gas.
The liquid may be sprayed in the form of a mist or atomized with larger-sized droplets or in the form of a continuous liquid.
In the thermal cycle that is effected, the cooling of the strip can then begin when the strip is at a high temperature, for example 750° C. When the strip is at a temperature that is much higher than the boiling temperature of the cooling liquid, a film boiling or vapor film situation occurs. This phenomenon is called calefaction. The layer of vapor causes something of a barrier to the transfer of heat between the strip and the water, thus reducing the effectiveness of the cooling with water.
For the example of water, the boiling temperature is close to 100° C. It can vary by a few degrees depending on the composition of the water and the quantity of additives in it.
In sum, in the situation of a vapor film (film boiling), the problem can be reduced to cooling an imaginary wall to 100° C. using water. The temperature of the atomized water is then a first-order parameter for controlling the intensity of the cooling, □=h (100° C.−T water ° C.).
In terms of the calefaction phenomenon, there is a critical temperature for the strip, known as the “Lindenfrost temperature”. For a temperature above this critical temperature, the cooling takes place with a vapor film and hence the cooling is ineffective but relatively very homogeneous. For a lower value of the temperature close to the critical temperature, the effectiveness of the cooling is significantly better but rather chaotic. In this case, there is a localized disappearance of the layer of vapor (the term “redampening” is then used), with a very high increase in the heat transfer. A steep temperature gradient results over the width of the strip, which can give rise to plastic deformations of the strip, for example the appearance of folds, or to heterogeneous mechanical properties over the width of the strip.
This critical temperature is dependent on numerous parameters, including the characteristics of the atomization, the temperature of the atomized liquid or the nature and temperature of the cooled surface.
The main factor is the effect on this temperature of the temperature of the cooling liquid and of the atomization parameters, ie the velocity and diameter of the droplets.
The object of the invention is especially to effect a homogeneous cooling of the metal strip, in particular to prevent the formation of folds or substantial differences in mechanical characteristics over the width and/or length.